The present invention is in the field of battery technology and, more particularly, in the area of solid polymeric materials and composites for use in electrodes and electrolytes in electrochemical cells.
Conventional lithium ion batteries include a positive electrode (or cathode as used herein), a negative electrode (or anode as used herein), an electrolyte, and, frequently, a separator. The electrolyte typically includes a liquid component that facilitates lithium ion transport and, in particular, enables ion penetration into the electrode materials.
In contrast, so-called solid-state lithium ion batteries do not include liquid in their principal battery components. Solid-state batteries can have certain advantages over liquid electrolyte batteries, such as improvements in safety because liquid electrolytes often contain volatile organic solvents. Solid-state batteries offer a wider range of packaging configurations because a liquid-tight seal is not necessary as it is with liquid electrolytes.
Generally, batteries having a solid-state electrolyte can have various advantages over batteries that contain liquid electrolyte. For small cells, such as those used in medical devices, the primary advantage is overall volumetric energy density. Figure For example, small electrochemical cells often use specific packaging to contain the liquid electrolyte. For a typical packaging thickness of 0.5 mm, only about 60% of the volume can be used for the battery with the remainder being the volume of the packaging. As the cell dimensions get smaller, the problem becomes worse.
Elimination of the liquid electrolyte facilitates alternative, smaller packaging solutions for the battery. Thus, a substantial increase in the interior/exterior volume can be achieved, resulting in a larger total amount of stored energy in the same amount of space. Therefore, an all solid-state battery is desirable for medical applications requiring small batteries. The value is even greater for implantable, primary battery applications as the total energy stored often defines the device lifetime in the body.
Further, solid-state batteries can use lithium metal as the anode, thereby dramatically increasing the energy density of the battery as compared to the carbon-based anodes typically used in liquid electrolyte lithium ion batteries. With repeated cycling, lithium metal can form dendrites, which can penetrate a conventional porous separator and result in electrical shorting and runaway thermal reactions. This risk is mitigated through the use of a solid nonporous electrolyte.
The electrolyte material in a solid-state lithium ion battery can be a polymer. In particular, poly(ethylene oxide) (“PEO”) can be used in forming solid polymer electrolytes. PEO has the ability to conduct lithium ions as positive lithium ions are solubilized and/or complexed by the ethylene oxide groups on the polymer chain. Solid electrolytes formed from PEO can have crystalline and amorphous regions, and it is believed that lithium ions move preferentially through the amorphous portion of the PEO material. In general, ionic conductivities on the order of 1×10−6 S/cm to 1×10−5 S/cm at room temperature can be obtained with variations on PEO based electrolyte formulations. The electrolyte is typically formulated by adding a lithium ion salt to the PEO in advance of building the battery, which is a formulation process similar to liquid electrolytes.
PEO has been widely studied as a component of solid electrolytes due to its comparatively high lithium ion conductivity. As one of the most extensively studied polymers, several reports combine PEO with lithium salts, plasticizers, and other fillers to make solid polymer electrolytes with comparatively high conductivity. One of the challenges for the implementation of PEO into lithium ion batteries is the instability of PEO at voltages higher than 4.2V. PEO will begin to degrade at these higher voltages, leading to a significant decrease in battery performance and ultimately limiting the number of cycles before cell failure.
However, solid-state batteries have not achieved widespread adoption because of practical limitations. For example, while polymeric solid-state electrolyte materials like PEO are capable of conducting lithium ions, their ionic conductivities are inadequate for practical power performance. Successful solid-state batteries require thin film structures, which reduce energy density, and thus have limited utility.
Depending on the specific components combined with the PEO into the membrane, the oxidation onset can be suppressed to higher voltages, thus improving the stability of PEO. Some literature has reported improved PEO stability based on specific components (see, e.g., M. Armand, Solid State Ionics, 9&10, 1983, 745). More generally, ionically conductive polymers like PEO have been disclosed with the use of a lithium salt as the source of lithium ions in the solid electrolyte. For example, Teran et al., Solid State Ionics (2011) 18-21; Sumathipala et al., Ionics (2007) 13: 281-286; Abouimrane et al., JECS 154(11) A1031-A1034 (2007); Wang et al., JECS, 149(8) A967-A972 (2002); and Egashira et al., Electrochimica Acta 52 (2006) 1082-1086 each disclose different solid electrolyte formulations with PEO and a lithium salt as the source for lithium ions. Still further the last two references (Wang et al. and Egashira et al.) each disclose inorganic nanoparticles that are believed to improve the ionic conductivity of the PEO film by preventing/disrupting polymer crystallinity. However, none of these formulations address all the limitations of solid electrolytes and provide the performance improvements seen in the embodiments disclosed below.